Linux full-disk encryption without initramfs

Are there any full-disk encryption schemes that can be done without an initramfs, rather getting the encryption key from the kernel cmdline? I know this sounds insecure, as an attacker could just read the bootloader files; but due this device's boot process, I have to manually enter the cmdline at every boot. I already compile my own kernels for this arm64 device, so custom kernel configuration options aren't a problem for me.